With advanced data processing techniques, there have been provided a variety of printing modes, which requires a method for expression of dots in which a print dot density can be changed with one and the same printer. For this purpose, it is necessary to provide a method of changing a record dot diameter. In the case of a laser beam printer, a method of changing a scanning laser beam power to thereby change a record dot diameter is well known in the art. For instance, Japanese Patent Application Publication No. 40742/1983 has disclosed a method in which an optical filter is disposed in the optical path of the output beam of a laser oscillator, to change the quantity of light and to thereby change the record dot diameter. However, in certain cases, specifically in a case where a suitable amount of exposure of the photosensitive drum to obtain predetermined print density is required, and where a correct amount of exposure of a print start position determining sensor are taken into consideration, the prior art optical filter method is problematic. Specifically, in order to maintain high print quality, the range is limited in which the quantity of light is allowed to change. In the case of Japanese Patent Application (OPI) No. 66465/1986 (the term "OPI" as used herein means an "unexamined published application"), the drive current of a semiconductor laser is adjusted to change the beam power, and to thereby change the record dot diameter. However, in this case, in addition to the above-described problem, another problem occurs. Specifically, in the adjustment of the drive current, the semiconductor laser oscillation threshold current changes with temperature, as a result of which the beam power is changed, and accordingly the record dot diameter is unstable. On the other hand, the record dot diameter changes substantially in proportion to the drive current when a heavy beam is used with low power. Therefore, the method raises another problem in that the record dot is low in sharpness. Furthermore, in the case where it is required to change the record dot diameter in a wide range, the aforementioned proportional relationship between record dot diameter and drive current is not established; that is, it is necessary to abruptly increase the drive current as the record dot diameter increases. This can be readily understood from the fact that the intensity of a laser beam assumes a Gaussian distribution.
In another example of the conventional record dot diameter changing method, a plurality of beams are combined to change the record dot diameter. For instance Japanese Patent Application (OPI) No. 164759/1982 employs an array laser; however, since a plurality of laser beams are employed, it is rather difficult to physically arrange the array laser.
In another example of a conventional record dot diameter changing method, a lens stop is provided in the light beam to change the beam diameter on the scanning surface. For instance, in the case of Japanese Patent Application (OPI) No. 31270/1988, a board having a plurality of holes different in diameter is disposed on the light source side of a rotary polygon mirror. However, the method is still disadvantageous in that the center of the beam must be in alignment with each of the holes formed in the board, a loss of power occurs because part of the beam is blocked, and a diffraction pattern is formed. Japanese Patent Application (OPI) No. 275214/1987 has disclosed a method of changing a beam diameter by using an electro-optical switch array and a polarizing plate; however, it is still disadvantageous in that the elements are expensive.
In another example of the conventional record dot diameter changing method, a plurality of optical paths are employed which provide different beam diameters on the scanning surface, and one of the optical paths is selected by a switching means (cf. U.S. Pat. Nos. 4,539,478 and 4,642,701). However, the method gives rise to another problem that, because of the plurality of optical paths, it is rather difficult to physically arrange the relevant components.
Furthermore, in another example of a conventional record dot diameter changing method, the beam diameter on the scanning surface is changed (cf. U.S. Pat. No. 4,651,169). In this method, the lens must be moved with high accuracy, and the optical axis must be positioned with high precision. Accordingly, it is considerably difficult to practice the method with a simple mechanism.